The development of semiconductor switching technology for high power applications in motor drive circuits, appliance controls and lighting ballasts, for example, began with the bipolar junction transistor. As the technology matured, bipolar devices became capable of handling large current densities in the range of 40-50 A/cm.sup.2, with blocking voltages of 600 V.
Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to the suitability of bipolar transistors for all high power applications. First of all, bipolar transistors are current controlled devices. For example, a large control current into the base, typically one fifth to one tenth of the collector current, is required to maintain the device in an operating mode. Even larger base currents, however, are required for high speed forced turn-off. These characteristics make the base drive circuitry complex and expensive. The bipolar transistor is also vulnerable to breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications, for example. Furthermore, it is difficult to parallel connect these devices since current diversion to a single device occurs at high temperatures, making emitter ballasting schemes necessary.
The power MOSFET was developed to address this base drive problem. In a power MOSFET, a gate electrode bias is applied for turn-on and turn-off control. Turn-on occurs when a conductive channel is formed between the MOSFET's source and drain regions under appropriate bias. The gate electrode is separated from the device's active area by an intervening insulator, typically silicon dioxide. Because the gate is insulated from the active area, little if any gate current is required in either the on-state or off-state. The gate current is also kept small during switching because the gate forms a capacitor with the device's active area. Thus, only charging and discharging current ("displacement current") is required. The high input impedance of the gate, caused by the insulator, is a primary feature of the power MOSFET. Moreover, because of the minimal current demands on the gate, the gate drive circuitry and devices can be easily implemented on a single chip. As compared to bipolar technology, the simple gate control provides for a large reduction in cost and a significant improvement in reliability.
These benefits are offset, however, by the high on-resistance of the MOSFET's active region, which arises from the absence of minority carrier injection. As a result, the device's operating forward current density is limited to relatively low values, typically in the range of 10 A/cm.sup.2, for a 600 V device, as compared to 40-50 A/cm.sup.2 for the bipolar transistor.
On the basis of these features of power bipolar transistors and MOSFET devices, hybrid devices embodying a combination of bipolar current conduction with MOS-controlled current flow were developed and found to provide significant advantages over single technologies such as bipolar or MOSFET alone. Classes of such hybrid devices include various types of MOS-gated thyristors as well as the insulated gate bipolar transistor (IGBT), also commonly referred to by the acronyms COMFET (Conductivity-Modulated FET) and BIFET (Bipolar-mode MOSFET).
Examples of insulated gate bipolar transistors are described in U.S. Pat. No. 5,273,917 to Sakurai; U.S. Pat. No. 5,331,184 to Kuwahara; U.S. Pat. No. 5,360,984 to Kirihata; U.S. Pat. Nos. 5,396,087 and 5,412,228 to B. J. Baliga; U.S. Pat. No. 5,485,022 to Matsuda; U.S. Pat. No. 5,485,023 to Sumida; U.S. Pat. No. 5,488,236 to Baliga et al.; and U.S. Pat. No. 5,508,534 to Nakamura et al. In particular, U.S. Pat. No. 5,360,984 to Kirihata discloses a semiconductor substrate containing an IGBT therein and a freewheeling/flyback diode for, among other things, bypassing parasitic reverse voltage surges which are typical in inductive power circuit applications. However, the antiparallel-connected freewheeling diode disclosed by Kirihata increases the area occupied by the IGBT and may cause an unnecessary stray inductance due to the wiring which interconnects the IGBT with the freewheeling diode. Moreover, the IGBT of Kirihata may be susceptible to sustained parasitic thyristor latch-up.
Thus, notwithstanding these attempts to form IGBTs, there still continues to be a need for methods of forming highly integrated power semiconductor devices comprising IGBTs which are capable of bypassing parasitic reverse voltage surges, typical in inductive power circuit applications, and have reduced susceptibility to sustained parasitic thyristor latch-up.